A conventional technology provided with a starter motor for starting a prime mover includes a technique disclosed in Patent Document 1. This conventional technique is provided with a primer mover, specifically an engine, a starter motor for starting the engine, a main hydraulic pump and pilot hydraulic pump drivable by the engine, a swing motor, specifically a hydraulic actuator drivable by pressure oil supplied from the main hydraulic pump, a directional control valve, specifically a flow rate control valve for controlling a flow of pressure oil to be supplied to the hydraulic actuator, and a pilot valve, specifically a control device for producing an output of a pilot control signal to switch the flow rate control valve by using, as a primary pressure, pressure oil delivered from the pilot hydraulic pump.
The conventional technique is also provided with a selector valve in a line, which communicates the pilot hydraulic pump and the pilot valve with each other, to prevent the output of the pilot control signal from the pilot valve. This selector valve is provided with a first select position and a second select position. At the first select position, the primary pressure from the pilot hydraulic pump can be supplied to the pilot valve. At the second select position, on the other hand, the supply of the primary pressure from the pilot hydraulic pump to the pilot valve is maintained disabled, and the pilot valve is brought into communication with a reservoir. The conventional technique is designed such that, when the selector valve is switched to the second selected position, the output of the pilot control signal from the pilot valve is prevented.
The conventional technique is further provided with a relay and a switch interposed between the relay and a power supply. The relay maintains the starter motor drivable while the selector valve remains switched to the above-mentioned second select position.
The conventional technique is also designed such that, when the above-mentioned switch is turned on with the selector valve having been switched to the second select position at which the supply of a primary pressure from the pilot hydraulic pump to the pilot valve is disabled, the starter motor is driven to start the engine.
Although not indicated clearly in Patent Document 1 referred to in the above, the conventional technique is designed such that fuel is supplied to the engine generally at the same time as the driving of the starter motor, and subsequent to low-speed rotation by the starter motor, the engine speed increases to such revolutions per minute as permitting driving the hydraulic actuator to perform work by the above-mentioned supply of fuel. In other words, the conventional technique is designed such that in a low-revolution state of the engine by revolutions of the starter motor, the driving of the hydraulic actuator cannot be realized to such extent as performing work.
In the state that the selector valve has been switched to the second select position at which the supply of the primary pressure from the pilot hydraulic pump to the pilot valve is maintained disabled, on the other hand, no pilot control signal is outputted to switch the flow rate control valve even when the pilot valve is controlled. Accordingly, the flow rate control valve is maintained neutral so that safety is maintained without causing any unexpected drive of the hydraulic actuator. When it is not desired to cause any unexpected drive of the hydraulic actuator as in the case that an operator moves away from the operator's seat, the above-mentioned selector valve can be switched to the second select position.
In a work machine to which the above-mentioned conventional technique is applied, for example, a hydraulic excavator illustrated in FIG. 2, it is a common practice at the time of maintenance to drain pressure oil which still remains in a hydraulic actuator, for example, a boom cylinder 34 for driving a front attachment 35, including a boom and the like. Described specifically, when the front attachment 35 of the hydraulic excavator stays standstill in the air as illustrated in FIG. 2, a pressure remains on the bottom side of the boom cylinder 34. Disconnection of a bottom hose from the bottom side of the boom cylinder 34 in this state without advance drainage or the like results in the spouting of oil, thereby contaminating the surrounding area. Moreover, such disconnection is dangerous, because the front attachment 35 suddenly falls down. For these reasons, it is necessary to drain the boom bottom pressure, that is, pressure oil from the bottom side of the boom cylinder 34 beforehand upon conducting maintenance such as the replacement of the bottom hose.
The work to drain the pressure oil, which still remains on the bottom side of the boom cylinder 34, is performed after lowering the front attachment 35, which is staying in the air, to the ground. As a result, a substantial amount of pressure oil still remaining in the boom cylinder 34 can be drawn out. The pressure oil which still remains in the boom cylinder 34 can be drawn out further by quickly moving a control lever of the control device alternately and repeatedly in normal and reverse directions.
As the technology for drawing out, as mentioned above, pressure oil still remaining in a hydraulic actuator, another conventional technique has been proposed as disclosed in Patent Document 2. This another conventional technique is constructed including special hydraulic equipment, such as an accumulator or a pressure oil drain valve, to drain pressure oil which still remains in the hydraulic actuator.    Patent Document 1: JP-A-6-49867    Patent Document 2: JP-A-7-238902